Low-frequency noise in a one-bucket test

[idif]

Hi.
I've just started working with a Biosemi EEG system we have in our lab for some time, and wanted to start from testing it, so I ran a one-bucket test for now for the external electrodes (That is, I used a bucket of water with salt, and submerged the CMS/DRL and all external electrodes in the water). Also canceled to filters as recommended. According to the user manual I should expect a flat line if everything is alright. However, what I actually got was a low-frequency drifts in all channels. Only when I turned the high-pass on-line filter on (cut-off of 6hz and maybe even less), I got the expected flat line.
What does it mean? I would be very helpfull for any help interpreting this.

Thanks,
Isaac

[Coen]

Normal electrode offset drift.

Best regards, Coen (BioSemi)

[idif]

I see. Can you please elaborate on its meaning? Does it imply something regarding the electrodes condition? Or is it a normal phenomenon normally corrected by using a high-pass filter?

Thanks!
Isaac